Electromechanical transducers



Filed Sept. 9, 1958 Attorney United States Patent Office 3,59,l3@ Patented Get. 16, i962 3,059,130 ELECTRGI'VECHANICAL TRANSDUCERS Robert Hill Robins, London, England, assignor to United Insulator Company Limited, Chessington, Surrey, England, a British company Filed Sept. 9, 1958, Ser. No. 759,992 Claims priority, application Great Britain Oct. 3, 1957 Claims. (Cl. 3109.6)

This invention relates to electromechanical transducers of the kind comprising a body of a piezo-electric material, or of a material or composition which has an effective piezo-electric response, carrying two or more electrodes, at least two of the electrodes being so arranged on opposed surfaces of the body that it functions wholly or in part as a thickness mode transducer. Transducers of this kind can be used to convert mechanical into electrical energy, or vice versa, and can also be used as filters, oscillators and for other purposes.

The invention is more particularly, but not exclusively, concerned with transducers based on ceramic bodies, having for an example an alkaline earth metal titanate as a principal constituent, but is also applicable to transducers based on bodies of other composition whether of hetero-crystalline or of single crystal form. For convenience bodies in which dimensional variation is so related to applied potential that they are suitable for use in electromechanical transducers will hereinafter be referred to as piezo-electric bodies whether or not they exhibit a true piezo-electric effect.

Objects of the invention are to provide methods of adjusting the resonant frequency of an electromechanical transducer, after adherent electrodes have been applied to the piezoelectric body of the transducer, and to provide electro-mechanical transducers which can readily be adjusted to increase their resonant frequency.

In the method in accordance with the invention of manufacturing and adjusting electromechanical transducers, a surface of a piezo-electric body, to which is to be applied an electrode which will function in conjunction with another electrode applied to the body as an electrode of a thickness mode transducer is, before the application to it of the electrode, formed with a groove or grooves, preferably of substantially uniform depth; the electrode is then applied to the grooved surface, either wholly or partly in the groove or grooves. Preferably two groups of grooves at an angle to each other are cut in such a way as to form a regular mesh pattern, preferably a square mesh, but any pattern formed from a single groove (e.g. a spiral) or from a number of grooves can be used, provided that the electrode material applied to the groove or grooves will cover a sufficient area and be sufficiently evenly distributed to function effectively as one electrode of the transducer and provided that, where more than one groove is used, the layers of electrode material in the various grooves are electrically interconnected, as by interlinking the grooves, or otherwise. The electrode can be applied by any process which will provide a sufiiciently adherent layer of metal for example a conventional silvering process. After the electrode has been applied to the groove surface of the body and a complementary electrode has been applied to an opposed surface, the frequency of the thickness mode transducer thus formed can be adjusted if required by removing part of the body from the lands projecting between the grooves, or between the parts of the single groove, as by grinding or lapping the grooved surface.

The electrode applied to the grooved surface is conveniently formed by depositing metal over the whole of that surface, i.e. covering the bottom and sides of the groove or grooves and the lands between the grooves or between the parts of the single groove. The removal of the metallic deposit from the lands during the first grinding or lapping has surprisingly little effect on the characteristics of the transducer.

When it is desired to provide for the possibility of obtaining a greater range of adjustment than would be possible if one electrode were applied to a grooved surface, both electrodes may be formed in this way. Alternatively one electrode may be applied to a grooved surface, as described above, and the other electrode may have an adherent layer of a glaze applied to it so that the frequency of the transducer can be increased by reducing the thickness of the glaze layer, for example by grinding, lapping or polishing.

The method of manufacture and adjustment in accordance with the invention has the advantage that the electrodes can be applied in their final form and location before the frequency adjustment is effected, thus enabling frequent or continuous monitoring of the frequency adjustment.

Two transducers in accordance with the invention and methods of adjusting the resonant frequency of these transducers will hereinafter be described by way of example with reference to the accompanying drawing in which FIGURE 1 is an elevation in cross-section of the first transducer before its frequency has been adjusted;

FIGURE 2 is a plan of the first transducer,

FIGURE 3 is an elevation in cross-section of the second form of transducer after its frequency has been adjusted, and

FIGURE 4 is a plan of a modified form of the first transducer.

Referring to FIGURES 1 and 2, the first transducer was made in the following way.

A disc 1 of ceramic material Was made by firing together ingredients which form a barium titanate ceramic containing 5% of calcium titanate and 0.5% of cobalt oxide (both percentages being based on the total weight of the barium titanate and calcium titanate), and both plane surfaces of the disc were ground flat giving the disc a thickness of approximately 12.7 mm. and a diameter of approximately 32 mm. Parallel grooves 2 and 3 were cut in one surface of the disc as shown in FIG- URE 2, the depth of the grooves being approximately 1.7 mm. and their width approximately 0.8 mm. The opposed surfaces of the disc were then silvered by a standard method to give a continuous metallic deposit 4 on the smooth surface and a continuous metallic deposit 5 on the grooved surface, after which the ceramic material was polarised.

Measurements of the piezo-electrical properties and other properties of the transducer were made by the normal radio frequency bridge technique and the thickness of the transducer was then reduced progressively by lapping the grooved surface, first to remove the metal coating from the lands between the grooves and then to remove the ceramic material from the lands.

The results of the electrical measurements are set out in the table below:

Thickness Coupling Capacitance Thickness resonant COGIllCiGnt @1Kc./s. Tan6 10- Ambient in mm. frequency (thickness in pF lKe./s. temp. C.

Kc./s. mode) 12. 76 20s 25 04.4. 81 20. 5 12. 71 20s. as 25 642 82 18.8 12. 50 209. 78 2o 20. 12. 23 212. 1c 29 633 82 21. 0 12. 00 21s. 26 30 631 93 21. 7 11. 70 220. 41 31 624 81 22.0 11. 46 224. so 27 615 81 22.

The first measurements (a) were made before the first ing between the grooves and by reducing the thickness reduction in thickness and the remaining readings (b)(g) of the glaze applied to the second electrode. were taken as the thickness was progressively reduced. 4. A thickness mode transducer comprising a piezo- From a comparison of measurements (a) and (b) it will electric body having two opposed major surfaces, one of be seen that removal of the metal coating from the lands which is grooved, and two opposed electrodes each adbetween the grooves has very little effect on the electroboring to and substantially coextensive with one of said mechanical properties of the transducer. The results show surfaces, the electrode on the grooved surface being 10- how the resonant frequency of the transducer can readily cated primarily in the bottom of the grooves which are be adjusted simply by lapping the grooved face of the sufliciently evenly distributed over said surface to entransducer. sure that the resonant frequency of the transducer can be Referring now to FIGURE 3, the second transducer adjusted by removing part of the body from the lands consists of a disc of ceramic material 6 identical with the projecting between the grooves. disc 1 being formed on its upper surface with grooves 5. A thickness mode transducer comprising a piezo- 7 in the same way as the disc 1. Both surfaces of the electric body having two opposed major surfaces, both of disc 6 were silvered originally in the same way as the which are grooved, and two opposed electrodes each adtransducer described with reference to FIGURES l and 2 0 hering to and substantially coextensive with one of said except that the silvering 9 on the lower surface is consurfaces, the electrodes being located primarily in the tinned part of the way up the cylindrical surface of the bottom of the grooves which are sufficiently evenly disdisc. The fiat surface of the silver coating 9 is protributed over said surfaces to ensure that the resonant vided with a layer of glaze 10, formed by painting on frequency of the transducer can be adjusted by removing to the silver suflicient of a finely ground ceramic enamel 35 part of the body from the lands projecting between the in a suitable organic vehicle to form a layer of vitreous grooves. structure 0.3 mm. thick when the layer is fired at 650 C. 6. A thickness mode transducer comprising a piezo- The resonant frequency of this transducer can be adelectric body having two opposed major surfaces, at least justed by lapping either or both surfaces, the frequency one of which is formed with a single continuous groove, change obtainable by lapping the glaze alone being of the and two opposed electrodes each adhering to and substanorder of 5 kc./s. tially coextensive with one of said surfaces, the electrode As shown in the drawing the silver coating on the lands on the grooved surface being located primarily in the between the grooves has been removed by lapping leaving bottom of the groove which is sufficiently evenly dis only coatings 8 in the grooves. tributed over said surface to ensure that the resonant fre- Referring to FIGURE 4, this figure shows the transquency of the transducer can be adjusted by removing ducer of FIGURES 1 and 2 modified in that the grooved part of the body from the lands projecting between the surface of the ceramic body 11 is formed with a single grooves. continuous spiral groove 12. 7. A thickness mode transducer comprising a piezo- WhatIclaim as my invention is: electric ceramic body having two opposed major sur- 1. A thickness mode transducer comprising a piezofaces, at least one of which is grooved, and two opposed electric body having two opposed major surfaces, at least electrodes each adhering to and substantially coextensive one of which is formed with a series of grooves of subwith one of said surfaces, the electrode on the grooved stantially uniform depth arranged in a regular mesh patsurface being located primarily in the bottom of the tern, and two opposed electrodes each adhering to and grooves which are sufliciently evenly distributed over said substantially coextensive with one of said surfaces, the surface t ensure th t th resonant frequency f th t electrode on the grooved surface being located primarily du can b adjusted by removing part of the body from in the bottom of the grooves which are sufficiently evenly the lands projecting between the grooves. dlstl'lbllifid over said SllI'fflCfi t0 BIISHIW? that the YesOnZ in- A method of manufacturing a thickness mode transfrequency of the transducer can he j l by removing ducer which comprises forming a transducer having a P Of the y from the lands prolectlng between the resonant frequency lower than the required value by formgroovesing grooves in at least one of two opposed major surtransducer m accofdance Wlth Clam; 1 fi faces of a piezo-electric body and applying two opposed i gnomes are arranged m two groups Sn S.tanna y at adherent electrodes to said body, each substantially corlght angles to each othe1 to form a substanually square mesh pattern extens1ve w1th one of sand surfaces w th the electrode on 3. A thickness mode transducer Comprising a piezcy the grooved surface located pr1mar1ly 1n the bottom of the electric body, a first electrode applied at least partly in groove and adluftmg the resonant frequency of the trans the bottom of grooves in a major surface of the body, f f by removmg P of the body from the lands P a second electrode applied to a smooth major surface of lechhg between the g the body ppgging aid grooved urface and an adherent 9. A I'l'lCthOd as claimed in claim 8 ill an electrode layer of a glaze on at least the major part of the exposed is applied to the grooved surface in such a way as to surface of the second electrode, the grooving being suflicover and adhere both to the bottom of the grooves and to ciently evenly distributed over said surface to insure that the surfaces of the lands between the grooves. the resonant frequency of the transducer can be adjusted 10. A method as claimed in claim 8 in which the resby removing part of the body forming the lands projectonant frequency is adjusted by simultaneously grinding away the coplanar surfaces of all of the lands between the grooves.

References Cited in the file of this patent UNITED STATES PATENTS 6 Gale June 6, 11950 Kettering et a1 Feb. 27, 1951 Bradfield Aug. 30, 1955 Harris Nov. 5, 1957 Rudnick Jan. 27, I959 Tibbetts et al Aug. 24, 1959 OTHER REFERENCES Cady: Piezoelectricity, by McGraw-Hill Book Co., published in 1946, p. 241, sec. 182. 

